This invention relates to internal combustion engines including control valves for idle air flow which are positioned by a stepper motor and further including means to arithmetically count the pulses to the stepping motor and thereby indicate the position of the valve. This type of valve position indicating apparatus might be termed a pseudo-position indicating apparatus, since it does not actually measure valve position but only indicates what the valve position should be as a result of the pulses received by the stepper motor. It may be possible for error to be introduced into such a position indicating apparatus if one or more of the pulses to the stepper motor are not counted or if a pulse counted by the counting means does not cause the valve to move. Therefore, it is desirable, from time to time, to initiate a reset operation in which the counter contents and the valve position are brought into accord.
If the stepper motor or valve has no actual position indicating potentiometer or similar apparatus, it is desirable to include a stop against which the valve can be closed and motor stalled in the closing direction to indicate a fixed reference position. If the valve is driven to this fixed position and the counter reset to a reference number such as zero, then the valve and counter means will be in accord. The valve can then be reopened to the desired position while the counter counts the opening pulses in the normal manner to indicate that position.
A typical electronic control for such apparatus might include a digital microprocessor such as the Motorola 6800 or its equivalent, which processes information in eight bit words. Thus, a position count might have any value between 0 and 255 in decimal notation for a total of 256 possible indicated positions. If the valve happens to be open to a position near its open limit, it will require anywhere up to 256 closing pulses plus the additional opening pulses before the reset operation is completed. In some applications, this might require as long as two to three seconds; and, during this time, the engine has no effective idle control by the valve. If the reset were initiated while the vehicle was in a standing idle mode and a heavy load exerted on the engine just after initiation thereof, the engine could easily stall before the reset was completed. In addition, it is undesirable to initiate the reset before the engine is started, since the voltage deliverable by a battery unassisted by an alternator before engine starting might be insufficient to guarantee actuation of the stepper motor by every pulse delivered thereto. There would be no point in initiating such a reset operation routine if one has no confidence that the valve and counter will actually be brought into accord.